Estrogen receptor-positive breast cancer is the most common type worldwide and is typically treated with endocrine therapy, which blocks or reduces the effect of estrogen. While these drugs are effective, tumors can develop resistance over time, prompting researchers to explore ways to extend treatment effectiveness. Diet, particularly controlled fasting, is emerging as a potential factor in enhancing therapy response.
Recent research suggests that short fasting periods may trigger hormonal and genetic changes in tumor cells, increasing their sensitivity to endocrine therapy. This discovery opens new avenues for combining dietary interventions with existing treatments.
Fasting Enhances Tamoxifen Effectiveness
Tamoxifen, a common endocrine therapy, works by blocking estrogen’s influence on tumor cells. Researchers found that severe and intermittent fasting can amplify tamoxifen’s impact by inducing genetic and hormonal changes in the cells. In animal experiments, tumors in mice that received both fasting and tamoxifen shrank significantly more than those treated with tamoxifen alone.
The key mechanism behind this effect involves epigenetic changes. Fasting alters the tumor epigenome—the chemical markers that regulate gene activity—effectively “re-educating” cancer cells. As a result, the cells become more treatment-sensitive and less capable of growing.
Glucocorticoid Receptor Plays Central Role
A major finding of the study is the role of the glucocorticoid receptor, which is activated by naturally occurring hormones such as cortisol. Fasting increases cortisol levels, which in turn activates this receptor in cancer cells. Activation switches on genes that suppress tumor growth while simultaneously reducing the activity of AP-1 proteins, which normally promote cancer cell proliferation.
When researchers removed the glucocorticoid receptor from tumor cells, fasting no longer enhanced tamoxifen’s effects, confirming the receptor’s central role.
Progesterone Receptor Adds Additional Protection
Fasting also increased progesterone levels in mice and in patients following a fasting-mimicking diet, activating the progesterone receptor—a factor that further inhibits tumor growth in estrogen receptor-positive breast cancer. While the study emphasized the glucocorticoid receptor, the progesterone receptor appears to contribute to the anti-tumor effects as well.
Fasting-Mimicking Diet Shows Similar Effects in Patients
In patients undergoing endocrine therapy, a brief fasting-mimicking diet produced comparable results. Blood tests revealed increased cortisol and progesterone levels, and tumor biopsies showed stronger activation of glucocorticoid receptor genes along with reduced markers of cell division. These findings suggest that even short-term dietary interventions can induce significant biological changes in tumors.
Steroid Drugs May Replicate Fasting Benefits
The researchers also explored the potential of mimicking fasting effects with drugs. Dexamethasone, a synthetic glucocorticoid commonly used to reduce inflammation, produced similar anti-tumor responses when combined with tamoxifen. Tumors shrank more, and the therapy remained effective for longer, suggesting a potential alternative for patients who cannot follow prolonged fasting routines. Safety studies would be needed before such treatments could be widely implemented.
Implications for Future Breast Cancer Treatment
This study highlights the potential of targeting the glucocorticoid receptor to enhance endocrine therapy in estrogen receptor-positive breast cancer. While fasting or steroid-based approaches show promise, further research is needed before clinical adoption. The findings provide a significant step toward overcoming treatment resistance, one of the biggest challenges in breast cancer therapy.
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